Method of cooling molten copper with a coolant flow velocity to exceed steam generation



TEMRERATURE INCREASE J'lilly 4 1957 D. B. coz-ER 3,329,197

METHOD OF COOLING MOLTEN COPPER WITH A COOLANT FLOW VELOCITY TO EXCEED STEAM GENERATION l Filed April 26, 1965 INCREASE HEAT TRANSFER* BTU/HR/SQ. FT.

cooLANT vELocTYFEE-r PER sEcoND WVM/TOR .fj/g 2 Danze] afer United States Patent Gli ice 3,32%',@7 Patented July 4, 1967 3,329,197 METHGD F COLING MULTEN CPPER WITH A COLANT FLOW VELCITY T@ EXCEED STEAM GENERATION Daniel B. Cater, Carrollton, Ga., assigner to Sonthrvire Company, Carrollton, Ga., a corporation of Georgia Filed Apr. 26, 1965, Ser. No. 450,855 7 Claims. (Cl. 16d-87) ABSTRACT 0F THE DISCLQSURE `What is disclosed is a method of cooling high melting temperature metals such as copper in a mold formed by a peripheral groove in a casting wheel and which includes flowing a coolant in each of a plurality of channels in the casting wheel at a velocity which prevents the formation of steam between the coolant and the mold. A coolant velocity of at least 16 feet per second is also disclosed as being that coolant velocity at which maximum heat transfer from the mold to coolant in the channels begins to occur. The method is disclosed in terms of coolant flowing in annular paths adjacent the peripheral groove and in terms of coolant flowing in paths along radii of the casting wheel.

This invention relates generally to metal casting and is more particularly concerned with methods of achieving improved productivity as to the rate of cooling the molten metal and achieving improved metallurgical qualities of cast products.

In the continuous casting of molten metal, as in the formation of continuous bars, rods, strips or sheets, the dissipation of heat is of particular concern both as to the quality of product and as to the rate of production. In the continuous casting of metals and alloys of a high melting temperature such as copper and steel, the problems of rapid, effective, efficient, uniform and controlled dissipation of heat becomes a critical problem.

While certain aspects of the present inventive concept may be broadly vapplicable in a Wide variety of casting processes in which molten metal of various melting and solidification temperatures are reduced to solid form, the present invention is here presented as related to the continuous casting `of high melting temperature metals such as copper by the use of a continuously rotating casting wheel in which the molten metal is supplied to a peripheral i mold groove to travel with the periphery of the wheel during the s-olidifc-ation of the metal and to be withdrawn from the peripheral mold groove when a self sustaining condition has been achieved by the metal as a result of reduction of the temperature of the nietalthrough forced convective heat transfer.

The problems involved in lachieving an optimum quality of metal and rate of production through controlled heat dissipation from the Imolten metal as it travels in the peripheral groove of the wheel are numerous. It is required that the metal be solidified to a self sustaining condition within less than a complete revolution of the casting Wheel since the metal must be withdrawn from the peripheral groove of the Wheel in order to vacate the groove of the wheel for the reception of a new supply of molten metal. Therefore, the rate of cooling the molten metal determines the rotational speed of the casting wheel which in turn is the controlling factor for production rate. In order to cool the high temperature melting metals to a self-sustaining condition and still maint-ain high production rates, then it is necessary that the maximum cooling rate of the molten metal be maintained at all times.

Moreover, the high temperature melting metals increase the temperature of the mold wall of the casting wheel so that the wheel is heated excessively unless maximum cooling of the molten metal and the mold Wall is maintained. Hence, for the protection of the casting wheel as Well as for enhancement of the rate of heat dissipation from molten metal in the peripheral groove, the maximum cooling rate within the casting wheel must be maintained.

Attempts to accomplish a more or less controlled, rapid, effective, and elicient heat dissipation of molten metal during its travel with the periphery of a wheel have resuited in arrangements for continuously spraying the periphery of the casting wheel with a coolant liquid to eifect forced convection. This method has not been found entirely satisfactory due to the lack of control and the inability of maintaining a uniform heat exchange between the metal and the coolant throughout the entire travel of the wheel from the point of molten .metal injection to withdrawal of the substanially solid product. Further, the quantities of coolant used in the cooling have been excessive, difficult to control and the maintenance of an adequately low temperature of the coolant has been expensive to achieve.

A more conventional method of heat dissipation for continuous casting wheels, and the molten metal therein, has been by the provision of annular coolant liquid passageways through which a coolant liquid is forced as the wheel rotates, to provide a forced convective heat exchange. Such heat exchange meth-od, and .apparatus therefor, are presently accepted practice, Difficulty arises, however, in the accomplishment of an adequate rate of heat dissipation, particularly in high melting point materials such as copper, by which adequate solidiiication may be insured during a rapid rotation of the wheel -as required for a desirable rate of production.

Factors controlling the rate of heat dissipation are manifold andintricate. For instance, as the molten metal solidies, a shrinkage takes place forming a gaseous gap between the exterior of the solidifying metal and the Walls of the mold cavity. Such intervening gap materially retards the rate of heat conduction from the solidifying metal to the material of the casting wheel, hence retards the rate at which heat is dissipated by the forced convection of coolant fluid. A further controlling factor in the rate of heat dissipation is the material of the wheel itself and the thickness of the walls of such material between the casting groove and the coolant channels of the wheel. A further factor which deter-s optimum eiciency of the coolant fluid is the generation of steam as a liquid coolant passes through the coolant channel adjacent the point of immediate entry of the molten material. Such steam forms a heat retarding barrier materially resisting optimum heat transfer effects. The material of the wheel also is of irnportant sgnicance.

It is therefore among the primary objects of the present invention to provide a novel and improved method for the achievement of an optimum rate of heat dissipation from the molten metal of a continuously rotating casting wheel.

Among the steps of the present method by which such optimum heat exchange is sought to Fbe achieved is the maintenance of a high rate of coolant liquid iiow through a coolant channel in a casting Wheel so as to substantially eliminate the thermal barrier effect of steam formation on the forced convection of heat from the molten metal.

A further means included in the present method is the provision of coolant channel walls of minimum crosssectional diameter thereby increasing the eiiiciency of forced convection heat dissipation from the molten metal.

A further and important provision of the present method is that of individualizing the flow of coolant through separate passages in heat conducting relation to the molten metal receiving channel so that the rate and volume of coolant liquid ilow and general heat dissipating character of the coolant is related to the area and mass of molten material exposed to the coolant. Selective control of coolant may be accomplished in relation to the requirements for heat dissipation of various surfaces of the molten material.

The use of a plurality of coolant passages also permits thinner wall thickness between the passages and thev metal receiving channel without deterioration of the strength of the wheel than would be possible with a single coolant passage. Since the present method overcomes the difficulties encountered using previous methods by providing for the maximum cooling of each of the wall sections defining the casting groove as well as the molten metal being solidified therein, casting speeds normally reserved for metals having low melting temperatures can be achieved for copper and other metals having high melting temperatures. As a result of the increased cooling rate, the outer mol-d wall temperature is reduced, thereby reducing the mean temperature of the mold wall sections to such an extent as to prevent any substantial deterioration of the mold wall sections even after extended periods of use.

The present method also includes the provision of a radial direction of movement of coolant fluid at individual sections of the wheel, thereby providing for an optimum rate of heat exchange, as related to the temperature of the metal, as the metal progresses in the travel of the wheel from its molten state to that of adequate solidication for withdrawal.

Numerous other features and advantages of the method of the present invention will be apparent from consideration of the following specification taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic fragmentary cross-section through a casting wheel graphically presenting the temperature distribution across the solidified metal within the casting groove of a casting wheel, the air gap, and the mold wall;

FIG. 2 is a curve indicating the general relationship between heat transfer and coolant velocity in apparatus of the type herein referred to;

FIG. 3 is a perspective sectional view of a casting wheel segment in which the present method is employed', and,

FIG. 4 is a cross-sectional view of a casting wheel segment employing a different form of the present method.

In considering the following specication, it is to -be understood that the curves, graphs, figures and formulae herein are by way of general illustration of theoretical considerations in this application and that the apparatus and method herein set forth in no way limit the present inventive concept. Widely varying conditions may widely vary the applicability of some of the data presented. The basic concepts, however, are among those which are necessary to achieving a full understanding of the present inventive concept.

FIG. 1 illustrates typical temperature drops that occur across the cooled mold wall 11, across the air gap 12 formed upon solidiiication shrinkage, and across the solidiied metal 13 during a continuous casting process of the type herein considered. Such temperature drops are due to the thermal resistances inherent in the mold wall 11, -the air gap 12, and the solidified metal 13.

Two basic areas of concern are characterized in FIG. 1: the heat to be transferred from the molten metal as the metal is solidified and cooled by the coolant C, and the anean temperature of the lmold wall itself. At the inter face between the metal 13 already solidified and the still molten metal M there is a temperature T1. Due to a temperature drop across the solidified metal 13, there is a temperature T2 at the outer face of the solidified metal 13 Likewise, due to the thermal resistance of the air gap 12, there is a temperature drop across the air gap which results in a temperature T3 at the inner surface of the mold wall 11. Due to the temperature drop across the mold wall 11, there is a temperature T4 at the outer surface of the mold wall 11. The temperature of the body of the coolant is T5.

It is commonly known in the casting art that there is less deterioration of the mold wall 11 as its temperature is lowered; therefore, it can easily :be seen that the temperature of the mold wall 11 should be maintained as low as possible. Those skilled in the art have generally considered the mean temperature the most indicative of the mold wall temperature. The mean temperature is determined by Tad-T4 It can easily be seen that the temperature T1 is substantially constant since the molten metal solidiies at a relatively constant temperature. This means that, since T1 is relatively fixed, T2 will also be relatively fixed due to the relatively constant thickness of the solidified metal 13 and its relatively constant thermal resistance. However, as the metal shrinks upon soliditication from a molten state, it forms the air gap 12 which is of a constant width because the metal will always shrink the same amount with the same change in temperature. This means that T3 is also a substantially fixed temperature since the thermal resistance of the air gap 12 remains substantially constant, causing the temperature drop across the air gap 12 to be substantially constant.

From the general heat transfer equation for a unit transfer of heat Ta-Tb R where:

q/A :unit transfer of heat,

Ta--temperature of the hot medium which is sought to be cooled,

Tb=temperature of coolant used to cool the hot medium,

and

R=total thermal resistance of the material separating the coolant from the hot medium,

it follows that, in order to increase the amount of heat transferred per unit time, Tb may be decreased, R may be decreased, or both Tb and R may be decreased at the same time. Due to the impracticability of reducing the temperature of the coolant, only the decrease of R is feasible.

R is represented by the heat transfer equation:

1 l la l l R= W 5 E m hco+kw+kaz+ksm hol where: hcc=average unit thermal convective conductance be- Since the unit conductances of the air gap 12, the mold wall 11, and the solidified metal 13 are substantially constant, and the widths of the air gap 12 and the solidified metal 13 are also substantially constant, the only quantities that may be feasibly varied so as to reduce the thermal resistance of the heat transfer system and to increase the quantity of heat transferred :by the system are hcu and lw.

Decreasing lW will decrease the resistance R directly proportionally. However, a minimum lW that may be used is reached since the mold wall 11 must be suciently strong to support the metal being cast therein. Therefore, the only practical way of increasing the heat transfer rate from the metal being cast is to increase hcc.

fr@C can be increased Iby increasing the flow velocity of the coolant past the mold wall 13 of the casting wheel. This can be seen by referring to FIG. 2 of the drawings. Note here that t-he value of hcc reaches a maximum, or substantially a maximum, value when the velocity is approximately 16-20 ft./sec. Velocities above this value do not appreciably increase the heat transfer through the mold wall 11; therefore, as long as the coolant velocity is maintained above approximately 16 ft./sec. the maximum cooling rate and, hence, the maximum casting rate will be realized. An important, if not controlling factor in the characteristic of the curve of FIG. 2 is the vaporization ofthe coolant fluid on the outer face of the wall 11. Such vaporization produces a substantial thermal barrier in the form of a steam layer. At -low coolant velocities, below 16 feet per second, for instance, the steam layer is permitted to build up, blocking heat transfer. At higher velocities, within limits, the vaporization does not form or is swept away to more effectively expose the wall 11 to heat transfer to the coolant.

In one successful application of the present method, the casting ring of FIG. 3 is used. The apparatus is more fully disclosed in the co-pending application, Ser. No. 413,930, filed Nov. 25, 1964. In such apparatus the casting ring 19 is the annular mold mem-ber of a casting wheel and defines a relatively deep but transversely limited, outwardly open casting `groove 20. A relatively narrow, transverse cooling fluid channel 21 is provided at the inner surface of the ring 19 spaced from the groove 20 -by a relatively thin mold wall section 22. Each side 24 of the ring 19 is formed with Wider radial cooling channel 25 spaced from the groove 20 by mold wall sections 26. Channels 21 and 25 are closed by fluid confining annular closure plates 27 and 28, respectively.

The mold wall 11 of FIG. 1 is representative of the mold wall sections 22 and 26 and represents the relatively static heat ilow conditions which exist during castring. The coolant such as water is separately introduced into the cooling channels 21 and 25 under sufficient pressure to cause the coolant to attain velocities in each channel 21 and 25 sufficient to prevent the formation of steam by the heat absorbed by the coolant while the coolant is passing through the channel 21 or 25, or sufficient to sweep away any coolant that is vaporized by heat absorbed from the` metal as fast as the coolant vapor is formed. Thus, as herein referred to, the thermal barrier produced by steam is either substantially eliminated or minimized. By reference to FIG. 2, it will be seen that such velocity is preferably greater than approximately 16 ft./sec. It will be also noted that the channels 21 and 25 provide individual coolant flow for individual mold wall sections 22 and 26. The cross-sectional areas of the cooling channels 21 and 25 are small enough so that the variations in coolant flow about the cross-section are not as pronounced as would be in a single channel providing the entire heat dissipation. Moreover, the plurality of streams of coolant allows each stream of coolant to be independently controlled by valve means (not shown) so that each stream may be varied in accordance with the desired amount of heat to be transferred through each mold wall section to achieve the desired cooling pattern within the cast metal. In order for velocities, in the range heretofore mentioned, to be developed, the streams of coolant are contained within the channels 21 and 25 so that sufcient pressure can be exerted on the coolant to force it through the channels at the desired velocities, thus creating a superior heat transfer relationship.

The apparatus shown in FIG. 4 for performing the method of the present invention is more fully disclosed in the co-pending application, Ser. No, 432,211, tiled Feb. 12, 1965. The casting ring 30 is the annular mold member of a casting wheel and defines a casting groove 31 6 of conventional configuration substantially similar in form to the groove 20 of FIG. 3.

In this form of the invention the substantially static heat characteristics of FIG. 1 are present as is the heat exchange and velocity relationship of FIG. 2. However, while FIG. 3 illustrates an annular division of coolant streams, such as provided by the central channel 21 and side channels 25, that form of the invention presented by FIG. 4 provides radially individual streams of coolant spaced arcuately about the wheel and in such manner that the flow of each individual stream is radially outward from the inner wall of the ring .30 and passing in heat exchange relation to the walls of the ring. While the concept of the present method is not necessarily so limited, the streams are also individualized as to the sides of the groove as well as to their radial flow. Thus, as shown in FIG. 3, the side walls 32 of the ring 30 are formed with circularly spaced radially extending uid passageways 34 defining a mold wall 35 against which the -coolant flows at the steam preventing or dissipating velocity, not less than approximately 16 ft./sec. as suggested by the heat transfer curve of FIG. 2. At its radially outward end each passageway 34 is turned outwardly as at 36 by the peripheral flange 37 at the mouth of the casting groove 31. To direct and separate the flow of the high velocity liquid directly against the side walls of the groove 31, intermediate posts 38 are provided, their inner faces 39 defining the outer walls of the radial channels 34 and their outer faces 40 defining the inner wall of inwardly directed discharge channels 41 formed in combination with the wheel side plates 42. Coolant fluid is supplied to the successive channels 34 by fixed supply nozzles 43 radially arranged about a circular coolant supply pipe 44. In the rotation of the casting wheel each successive coolant passage 32 registers with the successive nozzles 43 thereby allowing fluid to be supplied to the channels 34 for radial passage against the mold Wall 35 at the required velocity for optimum cooling. Since, in the travel of the wheel, the requirements for heat transfer will be greatest as each mold section receives the molten metal, the requirement diminishing therefrom to the point of finished material extraction, the flow velocity and/or volume may be varied by variations in size or orifice of the individual fixed nozzles to meet these requirements. Thus, in the apparatus of FIG. 4, the flow is of the required velocity to exceed steam generation and is variable in its individual streams to meet varying requirements for heat exchange of varying areas of the wall.

It will be obvious to those skilled in the art that many variations may be made in the embodiments chosen for the purpose of illustrating the present invention without departing from the scope thereof as defined by the appended claims.

I claim:

1. In a method of cooling molten copper in a mold formed by a casting wheel having a peripheral groove in which molten metal is received, the steps of transferring heat to said mold from a molten metal in said peripheral groove, and transferring heat from said mold to a coolant owing through a plurality of channels in said casting wheel adjacent said molten metal, said coolant having a velocity of flow through each of said channels which is greater lthan that velocity at which the total heat transfer from said mold to said coolant while said coolant is flowing through a channel results in the formation of steam in said channel between said coolant and said mold.

2. The method of claim 1 including flowing coolant through said plurality of channels along annular paths.

3. The method of claim 2 including varying the velocity of coolant flowing through one of said plurality of channels independently of the velocity of coolant flowing through another of said plurality of channels.

4. The method of claim 3 in which said velocity of coolant flowing through one of said plurality of channels is at least 16 feet per second.

5. The method of -clairn 1 including flowing coolant through said plurality of channels along radii of said casting wheel.

6. The method of claim 5 including varying the velocity of coolantowing through one of said plurality of channels independently of the velocity of coolant owing through another of said plurality of channels.

7. The method of claim 6 in which said velocity of coolant ilowing through one of said plurality of channels is at least 16 feet per second.

References Cited UNITED STATES PATENTS 2,865,067 12/1958 Properzi 2257.4 2,946,100 7/1960 Baier et al. 22-212 3,151,366 l0/l964 Fromson 22-200.l

FOREIGN PATENTS 336,556 4/1959 Switzerland.

10 I. SPENCER OVERHOLSER, Primary Examiner.

R. D. BALDWIN, Assistant Examiner. 

1. IN A METHOD OF COOLING MOLTEN COOPER IN A MOLD FORMED BY A CASTING WHEEL HAVING A PERIPHERAL GROOVE IN WHICH MOLTEN METAL IS RECEIVED, THE STEPS OF TRANSFERRING HEAT TO SAID MOLD FROM A MOLTEN METAL IN SAID PERIPHERAL GROOVE, AND TRANSFERRING HEAT FROM SAID MOLD TO A COOLANT GLOWING THROUGH A PLURALITY OF CHANNELS IN SAID CASTING WHEEL ADJACENT SAID MOLTEN METAL, SAID COOLANT HAVING A VELOCITY OF FLOW THROUGH EACH OF SAID CHANNELS WHICH IS GREATER THAN THAT VELOCITY AT WHICH THE TOTAL HEAT TRANSFER FROM SAID MOLD TO SAID COOLANT WHILE SAID COOLANT IS FLOWING THROUGH A CHANNEL RESULTS IN THE FORMATION OF STEAM IN SAID CHANNEL BETWEEN SAID COOLANT AND SAID MOLD. 